System for true amplitude modulation of radio waves



April 13, 1.948. D. B. KEI-:VER

SYSTEM Fon TRUE `AMPL1TUDE MoDULATioN oF Alamo wAvE Filed oct. 1v, A1947 2 sheets-sheet 1 Inventor omzfz/wf A tto meys .www

April 1,3,` 1,948.`

SYSTEM FOR TRUE AMPLITUDE MODULATIONOF A RADIO'WAVE Filed Oct. 17, 1947 2 Sheets-Sheet .2

I uventnr v Atto rneys Patented Apr. 13, `1948j illfblilf'l'ED STATES PATENT OFFICE SYSTEM" FORV TRUE AMPLI-TUBE' MODULA- TIO'N 01""RADIO"VVAVFSz I Donald B. Keever, Colfax,.N. C.

Application October I7, 1947', Serial No. 780,315

10A Claims. 1

The present invention relates to modulation systems and it consists in the combinations, vconstructions and arrangements of parts herein described and claimed.

An object of the invention is to reduce the channel width required for the transmission of an amplitude-modulated wave..

A further object of the invention is the reduction of the required channel width by the substantially complete elimination of side bands leaving only a wave of varying amplitude and of single constant frequency.

Other and further objects will become apparcnt upon reading the following specification together with the accompanying drawings forming apart hereof.

Referring to the drawing:

Figure 1 is a symbolic block diagram of an embodiment of the invention, and

Figure 2 is` a schematic circuit diagram illustrating an*` arrangement in accordance with the diagram of-Figure 1.

Referring to `Figure l, the modulating frequencies are shown as originating at a microphone II and are applied to the input of a. speech amplier I2. Any other suitablesource of modulating frequencies may be used,` however. The output of the amplifier Ii.` is connected to a modulator I3 which is connectedr to produce amplitude modulationA of carrier currents passing through modu-` lated amplier` Ill; The` carrier currents are derivedfrom a stable oscillator I5 or any other suitable source of constant frequency carrier current,

The amplitude-modulated output of amplier I4, together withE the usual side-bands resulting from the modulation is applied to a further amplifier I6 whose characteristics are substantially linear; The output of amplifier I6 is divided into two portions, one of which is passed through a limiter I1 indicated as being of the clipperjtype.` The limiter Il removes the amplitude modulations leaving a modulated current of constant amplitude which varies in frequency in accordance with theA side-bandv frequencies;

Thev discriminator ISis of the usual type and the magnitude `of the output voltage thereof varies in accordance with the amount of deviation of they frequency from a predetermined center frequency of the instantaneous phase relationships being determined by the direction ofsuch deviation.

Connected to the output of the discriminator Iifisl an audio frequency'amplifier I9 whose output` is suii'icient tomodulate` amplifier 2t to thesame degree asthe degree of modulation produced 2 at' the amplifier I4; Modulated amplifier 2U is substantially, identical inall respects' to the amplifier I4 and modulates the output of stable oscillator 2I. The stable oscillator 2li, except for its output frequency, is identical to ther stable*` oscillator I5. Both oscillatorsmay be ofD different harmonics selected fromy a common frequency the differencefrequency, ifV required. The-filter 23 will` not be necessary; however, if the chaiu acteristics of `linear driver stage '24' and power aml plier 25 are suciently selective.

The driver stage 24 andA power amplifier 25 are of conventional' type and serve to provide the desired amount of outputenergy for radiation at the antenna 26 or for' transmission by any othe desired means.. Y

The amplifier" Ieis provided with a gain control so that its amplification may be adjusted for balancing. purposes.

Referring' to Figure 2 in which the circuits of the various elements' shown in Figure 1 areill'us-` trated inigrea-ter detail, the stable oscillator I5 is shown byway of `example as comprising a crystal' I5-IJ?I connected to a triode l5-U2 to provide a crystalcontrolled oscillator of conventional' type.

The oscillator 2 Iissimilar in all respects tothe oscillator'IS except for the frequency 'of opera'- tion.

The microphone I I is connected to the amplifier IZ whichI comprises a triod'e I Z-III associated with a volume control I'202 in conventional manner to amplify the currents generated in the microphone II`.

The outputof the'speech amplifier I2 is shown capacitatively coupled to a further amplifier' or modulator stage I34 which raises the level to a point sufficient toproduce plate modulation in stage I4; The modulator I3' is shown as comprising a tetrode I3`III coupled by a transformer I3-02'to the modulated amplifier |41 The amplier I'lI is illustrated as comprising a tetrode Id-II'I operatively associated with a tuned. outw put transformerV I4-02` toA which the. modulated anode potential'from" the transformer I3-II2"'is`V applied. A desired amount ofmodulated' outr put voltage is taken from an adjustable tap I4-03 and applied through a coupling capacitor Iii-02 to the control grid of tetrode I6-0I of the linear amplifier stage IB.

The ,tuned output transformer Iii-413 is shown provided with two secondaries I6-IJ4 and IIE-05. The secondary I 6-04 is directly connected to the input of the mixed 22 which is shown as being of the balanced type.

The other secondary is connected to the input transformer I'I-03 of the clipper limiter stage I'I which comprises the two oppositely connected diodes II-ill and II--02 which are connected in multiple and in series with suitable sources of biasing potential indicated as batteries Il and I1-05. For symmetrical limiting, the potentials of batteries I'I`-04 and IT-IJS are substantially equal and they are applied to their respective diodes with a polarity which will prevent conduction until the biasing potential is exceeded. When the instantaneous voltage in either direction exceeds the biasing potential of the corresponding diode, conduction will take place and the excess voltage will be limited by current flow through one of the diodes.

The output of the limiter I1 passes to the input of the discriminator I8 which is of the conventional Travis type and comprises the diodes I8-02 and I8-03. This discriminator, which is a well-known device in the art of frequencymodulation, converts changes in frequency into changes in amplitude, and these in turn are passed on to the tetrode amplifier stage I9 which is similar to the amplifier stage I3, its output being sufficient to plate modulate stage 2U,

The output of the oscillator 2|, after modulation in stage 20, is applied to the center tap connection 220I of the balanced mixer 22 where it is combined with the output of the amplifier I6. The output of the mixer 22 is connected through the filter 23, if used, to a concentric transmission line 30 to the driver stage 24 and the power amplifier stage 25 from which the signals are applied to the antenna for radiation.

The circuits shown for each of the various stages are conventional in character and are well-known in the art and other equivalent circuits of similar character may be selected as desired to meet any particular set of operating conditions encountered in practicing the invention.

In operation, the modulated output of amplifier I 4 is carried through to the mixer 22 andconsists of the carrier frequency generated by the oscillator I5 together with the side-band frequencies produced by the modulation. A portion of the carrier frequency of the oscillator I5 together with the side-band frequencies is tapped off at the amplifier I6 and is limited in the clipper I1 so that the amplitudemodulation is substantially completely eliminated, leaving a signal modulated in frequency in accordance with the side-band frequencies.

The discriminator I8 has its center frequency adjusted to coincide with the frequency of the oscillator I5 so that the deviation from the carrier frequency is the factor which causes an output to be obtained from the discriminator, the magnitude ofthe output being substantially proportional to the amount of such deviation, but the particular frequencies remaining unchanged. 'Ihe current thus produced is modulated upon a carrier frequency derived fromthe oscillator 2I and the modulated output contains side-bands together with the carrier frequency which are applied to the mixer 22.

In the mixer 22, the two frequencies produced by the oscillators I5 and 22 are combined together with the modulations impressed thereon. One of these will consist of the usual amplitude modulated carrier and the side-bands. The otherwill consist of modulation products whose instantaneous magnitudes are in direct proportion, not to the instantaneous magnitudes of the other sideband components, but to their frequency deviation-sfrom the carrier. By applying these components in such phase relationship as to cancel the side-band components of the direct channel from amplifierV I6, and by adjusting their relative magnitudes by means of the amplifier adjustments, the side-band frequencies will be cancelled out leaving only a true amplitude modu- V lated carrier without the side-bands.

What is claimed is:

1. A modulation system comprising a source of signalling currents, a first source of carrier frequency, modulating means arranged to modulate the first carrier source with the signalling currents, limiting means responsive to a portion of the modulated currents and arranged to remove amplitude modulation therefrom, frequency discriminating means responsive to the limiting means and disposed to produce an output whose magnitude is substantially proportional to the frequency deviation of the limited currents from the first carrier frequency, a second source of carrier current, further modulating means arranged to modulate the current of the secondv source with the output currents of the discriminating means, and mixing means disposed to combine the currents of the two modulating means in such degree and in such phase relation-- ship that a modulated current is produced consisting in its entirety of a third carrier frequency modulated in amplitude and unaccompanied by extraneous currents of side-band frequencies.

2. A modulation system as in claim l, in which the third carrier frequency consists of the sum of said first and second carrier frequencies.

3. A modulation system as in claim 1 in which the third carrier frequency consists of the difference between the first and second carrier frequencies.

4. The method of obtaining a current signal modulated in amplitude alone without accompanying side-band frequencies which consists in the steps of generating a first carrier current of substantially constant frequency, modulating this current with the signalling current, taking a portion of this modulated current and removing the amplitude variationstherefrom, attenuating the last-named currents inversely in accordance with their frequency deviations from y frequency of the first carrier current, generating a second carrier current of substantially constant frequency, modulating the second carrier current with the attenuated currents, and combining the modulated first and second carrier currents in such degree and in such phase relationship that the combined current consists of a current of a third frequency modulated in amplitude alone and unaccompanied by extrane-V currents from the rst carrier frequency, a second source of carrier current, further amplitude modulating means arranged to modulate the cur- Irent of the second source with the output currents of the discriminating means, and mixing means disposed to combine the currents of the two modulating means in such degree and in rent is produced consisting in its entirety of a third carrier frequency modulated in amplitude and unaccompanied by extraneous currents of side-band frequencies.

8. A modulation system comprising a source of signalling currents, a first source of carrier frequency, amplitude modulating means arranged to modulate the first carrier source with the sigsuch phase relationship that a modulated curnalling currents, clipper type limiting means responsive to a portion of the modulated currents and arranged to remove amplitude modulation therefrom, frequency discriminating means responsive to the limiting means and disposed to produce an output whose magnitude is substantially proportional to the frequency deviation of the limited currents from the first carrier frequency, a second source of carrier current, further amplitude modulating means arranged to modulate the current of the second source with the output currents of the discriminating means, and balanced mixing means disposed. to combine the currents of the two modulating means in such degree and in such phase relationship that a modulated current is produced consisting in its entirety of a third carrier frequency modulated in amplitude and unaccompanied by extraneous currents of side-band frequencies.

9. A modulation system as in claim 8 in which the third carrier frequency consists of the sum of said first and second carrier frequencies.

10. A modulation system as in claim 8 in which the third carrier frequency consists of the difference between the first and second carrier frequencies. r

DONALD B. KEEVER.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 1,624,498 Mohr Apr. 12, 1927 2,377,326 Crosby June 5, 1945 

